As it was a bottle episode, the episode did not use any additional sets or guest stars with the intention of cutting down on costs for the series. This also gave time for the visual effects team to finish earlier work. The episode received mixed reactions from critics, with praise directed at the links to The Original Series, while criticism directed towards some of the characterization of the characters. "Observer Effect" was watched by 2.76 million viewers on first broadcast, which formed part of an overall drop of viewers for the series.

The aliens take a keen interest examining the human response to this crisis, and compare notes to previous reactions by Klingons and Cardassians. It is also clear they are members of an advanced species looking to make "first contact". So far, based on 10,000 years of observations, no species has been deemed ready. Seeking a different view of the crew, they temporarily shift to the bodies of Phlox, T'Pol, and Archer. A difference in opinion between the two aliens starts to form: one seems determined to maintain their non-interference protocol, while the other feels the protocol is outdated and unnecessary.

With time running out, Phlox and T'Pol find a way to disrupt the virus using deadly levels of radiation. Archer and Phlox, while wearing environmental suits, escort Tucker and Sato to Sickbay for treatment. Sato soon goes into cardiac-arrest, and Archer removes his gloves and helmet to assist her, but she cannot be resuscitated. They then administer a dose of radiation to Tucker, but that too is ineffective. Phlox then leaves to resume work from the Bridge. Suddenly, Tucker and Sato are reanimated by the aliens, who – while possessing the crewmen – explain the situation to a surprised Archer, who then makes an impassioned speech on behalf of his crew. The aliens decide to modify their procedures, choosing to resurrect and cure the infected crew members, when they previously would have left them to die. The aliens erase the encounter from crew's memory. Archer orders a warning beacon to be placed above the planet, and the aliens leave to begin planning first contact with more advanced humans at a later time.

This was one of show runner Manny Coto's deliberate moves to link Enterprise closer to The Original Series during the fourth season. It was the second time that the crew of the Enterprise (NX-01) had met non-corporal entities following the second season episode "The Crossing". Filming began on the episode on October 22, 2004, and continued for the following seven working days. While the makeup department needed to present Park's and Trinneer's characters as they progressed through the sickness introduced by the Organians, the visual effects team was freed up to complete production on earlier episodes.[1]

"Observer Effect" was first broadcast in the United States on January 21, 2005, on UPN.[4] It was watched by 2.76 million viewers,[5] which was a decrease from the 3.03 million who watched "Daedalus" but more than the 2.53 million viewers for the following episode, "Babel One".[6][7] Writing for The A.V. Club in 2014, Alasdair Wilkins included "Observer Effect" in his list of the twenty best episodes of the series.[8]

Jamahl Epsicokhan, at his website Jammer's Reviews, described the plot of "Observer Effect" as not being as predictable as it might initially seem due to the crew's inability to save Tucker and Hoshi and their reliance on the aliens to do so. He felt that the links to "Errand of Mercy" were both "subtle" and "sublime",[9] while describing the overall bottle show as "No slam-bang excitement; just a commitment to observation and plausible procedure."[9] He adds that the episode was an example of "humanist science fiction" rather than an "adventure show".[9]

Michelle Erica Green disliked the episode, writing about it in a review for TrekNation. She called it "clichéd, predictable and boring",[10] saying that the sudden differences in characterization in this episode in this episode, compared to how those characters acted in earlier appearances, could potentially lead to confusion as to when they were actually controlled by the aliens. She criticized the plot which only affected the senior crew members on the ship and said that it had "ripped off" several prior episodes of the franchise including "The Empath",[10] "Homeward" and "Scientific Method".[10]

"Observer Effects" was released on home media in the United States on November 1, 2005, as part of the season four DVD box set of Enterprise.[11] The Blu-ray edition was released on April 1, 2014.[12]